Conventionally, in vehicles, particularly in small two-wheeled vehicles and the like, ACG (AC generator) starter motors acting as starter motors at the start of engines and acting also as power generators after the start of the engines are often used (see, for example, Patent Document 1). Three-phase DC brushless motors are used for such ACG starter motors. For detection of a rotor position at the start of the engine and at the time of power generation of this three-phase DC brushless motor, a hall sensor has been provided for each winding of each phase of the three-phase winding, and thereby a rotor position has been detected using the hall sensor.
In the case where a hall sensor is used to detect a rotor position, however, when the hall sensor is arranged for the winding of each phase, it is necessary to arrange it for the purpose of accurately detecting the rotor position. For this reason, a high mounting accuracy is required for the positioning in arranging the hall sensor for the winding of each phase. Therefore, a working process of manufacturing the ACG starter motor becomes time-consuming by the work of mounting the hall sensor, thereby causing an increase in price of the three-phase DC brushless motor, such as the ACG starter motor described in Patent Document 1. Further, since the price of the hall sensor itself is high, the price of the ACG starter motor is increased.
In view of this, a technique of detecting a rotor position without using the hall sensor has been developed (see, for example, Patent Document 2). This Patent Document 2 discloses an example of a control device for a sensorless brushless motor. The control device described in Patent Document 2 detects a rotor position of the brushless motor by the following three methods. Firstly, when the brushless motor is stopped, a DC voltage with a plurality of patterns is conducted to two windings of the three-phase windings for a short period of time, and rising characteristics of current which varies in accordance with the rotor position are detected, thereby detecting the stop position of the rotor. Secondly, when the brushless motor is driven by a 120° conduction method, zero-cross points of the non-conduction phase are detected, thereby detecting the rotor position. Then, thirdly, when the brushless motor is driven by a 180° conduction method, the rotor position is detected based on a voltage induced in a sub coil attached to the winding of one phase.